44 PEACTICAL PHYSIOLOGY 



contraction begins, and the line joining the apices and basis of the 

 successive contraction ascends slightly. With 20 stimuli per sec. the 

 summation and fusion of each individual contraction is more complete > 

 but the apex of each individual contraction will probably still be seen : 

 the curve is therefore one of incomplete tetanus (Fig. 48). With 30 

 stimuli per sec. fusion may be complete from the first, i.e. complete 

 tetanus, or if not complete at first, it gradually becomes so. This 

 gradually increasing fusion (Fig. 48) is really due to fatigue : for 

 the period of relaxation of the individual contraction tends to become 

 longer and longer, and therefore the next stimulus reaches the muscle 

 progressively earlier in each individual twitch, until a point is reached 

 in which there is no time for the muscle to begin to relax between the 

 stimuli, and fusion becomes complete. With the Wagner's hammer, which 

 causes the muscle to receive 50 or more stimuli per sec., fusion is com- 

 plete from the first. One other point is to be noted in nearly all these 

 curves : at first the rise in the lever is very rapid, then it suddenly 

 becomes more gradual, but, even when fusion has been complete from 

 the first, the lever may still rise slowly for a short time until the muscle 

 has reached the utmost shortening of which it is capable. If the 

 stimulation is still continued, this height may be maintained for a short 

 time, but sooner or later the lever will begin to drop, showing the onset 

 of marked fatigue. In all cases when the stimulation ceases, the 

 relaxation is at first extremely rapid, then becomes more gradual and a 

 1 contraction-remainder ' varying in extent according to the degree of 

 fatigue is generally seen. 



The same experiments may be performed with a hyoglossus prepara- 

 tion. This muscle, however, being of the * granular' variety and 

 having a contraction which lasts twice as long as that of the ' clear 

 gastrocnemius (see p. 30), is sent into complete tetanus with half the 

 number of stimuli, i.e. about 15 per sec. 



CHAPTER VIII. 

 THE PROPERTIES OF NERVE, MINIMAL AND MAXIMAL STIMULI. 



A NERVE is not a unit ; it is that branch of a nerve-cell which conducts 

 an impulse to, or from, the periphery. A nerve-cell with its dendrites 

 and axis-cylinder process or axon forms a unit, the neuron. It is con- 

 venient, however, to examine the characteristics of a nerve apart from 

 its nerve-cell. The chief of these are excitability and conductivity. 

 Excitability, or, as it is sometimes called, irritability, is the response to a, 



